The objectives of this study are to (1) assess the efficacy of monthly MEP IGIV therapy in reducing the frequency of acute pulmonary exacerbations over one year in patients with cystic fibrosis; (2) to assess the effect of this therapy on the rate of decline in FEV1; (3) to assess the safety of monthly MEP IGIV therapy over one year in these patients; (4) to obtain a population-based assessment of MEP IGIV therapy.